Reporting: Make DeliveryAgent self-scheduling

Reporting is a spec for delivering out-of-band reports from various other parts of the browser. See http://wicg.github.io/reporting/ for the spec, or https://goo.gl/pygX5I for details of the planned implementation in Chromium.

This makes the existing DeliveryAgent class self-scheduling using the ReportingObserver interface: when reports are added to the cache, or periodically while reports are still queued, it automatically runs the "send reports" algorithm described in the spec.